The proper operation of a laser requires that the divergence of the oscillating beam be within acceptable bounds at any given point within the laser cavity. If the beam divergence is outside of these bounds, the cavity becomes unstable and the performance of the laser is severely degraded.
In the process of pumping the gain medium of a laser, heat is generated. This heat results in a nonuniform temperature distribution within the gain medium and a phenomenon called thermal lensing occurs. Thermal lensing is an optical distortion caused by the combination of a nonuniform temperature distribution and a temperature dependent index of refraction, thermal expansion, or both. For many laser systems, the thermal lens in the gain medium is radially symmetric about the axis of the laser, and, to first order, acts as a parabolic lens. As the laser is pumped harder, the strength of the thermal lens increases, changing the divergences of the oscillating beam as it exits the gain medium. The laser cavity is therefore stable over a limited range of operating powers.
Various attempts have been made at solving the thermal-lensing problem. One solution involves a telescopic zoom lens comprising a pair of short focal length lenses of opposite optical power. This system compensates for the thermal lensing of the gain medium by mechanically adjusting the spacing between the lenses, maintaining the beam divergence within acceptable limits. The high cost and mechanical complexity of this system render this solution unattractive.
Another solution utilizes a compensator comprised of a cylindrically shaped body of optical material with a temperature-dependent index of refraction thermally coupled to a controlled heat source at the perimeter. When the temperature of the perimeter is changed, a transient nonuniform temperature distribution is induced within the compensator, resulting in a thermal lens which can be made to compensate for the pump-induced thermal lens in the laser gain medium. The problem with this system is that it relies on a nonequilibrium temperature distribution within the compensator material. When the temperature of the perimeter stops changing, the optical material eventually reaches thermal equilibrium, reducing or eliminating the corrective power of the compensator. The compensator must be cycled in temperature before the desired results can again be achieved.